(1) Field of the Invention
The present invention relates generally to industrial wood working machines, more particularly, to large diameter cutterheads for peripheral milling machines, including high speed planers and molders.
(2) Description of the Prior Art
In wood planing applications, the need exists for cutterheads which can take advantage of the availability of new, modern day cutting materials while, at the same time, being simple and economical to maintain preferably on site with simple, inexpensive grinding equipment. In certain cases, particularly in "dimension" lumber operations, cutterheads are required to produce rounded edges on lumber of different thickness. This has typically required different cutterheads and the attendant stockpiling of a variety of cutterheads.
A wide variety of cutterhead designs exist for the wood molding industry where a multitude of different shapes are encountered. However, molding cutterheads are not used for large diameter, high speed planers used in the dimension lumber industry due to the relatively small number of knife rows possible, limitations as to cutterhead strength, and maintenance considerations Accordingly, high speed wood planers typically utilize straight knife-type tool steel or carbide tipped insert-type knives for the top and bottom heads which may be a staggered tooth or helical design. Straight knife-type tool steel heads or stacked saw-type carbide tipped profile heads are used for the side heads which often require a profile-type knife design to produce rounded edges.
Tool steel knife and continuous edge carbide knife designs have a maximum knife row capacity of about 20 for a 13 inch diameter cutterhead. Inserted tooth cutterheads and solid-type cutterheads with machined pockets for brazed on teeth are usually a staggered tooth design which requires additional knife rows since the passage of two rows is required to make a completed cut. A solid head body design with pockets for brazed on knife edges would not be limited to 20 knife rows for a 13 inch diameter cutterhead. While this design is used in some small cutterheads, it is not practical for large diameter cutterheads since it has no provision for tip replacement or profile adjustment, and has no flexibility regarding the variety of shapes produced or cutting widths obtainable or knife configuration, e.g. straight or staggered.
Switching out of entire sets of cutterhead knives when damage occurs to the cutting edges is well known for the straight knife-type tool steel top and bottom and side cutterheads. However, replacement of damaged knives is not possible for solid-type cutterheads. For high speed four sided planers, an additional and sometimes conflicting requirement includes the need to control or limit the noise level while maximizing the number of knife marks per inch. Knife marks per inch (kpi) are an indicator of surface finish quality and are given by the formula: kpi=(# of knife rows * RPM)/feed speed (ipm). For most high speed planers a 5-10 kpi surface finish is satisfactory. For example, for a feed rate of 12,000 ipm (1000 fpm) and 3600 RPM, approximately 32 rows of knives are needed to produce a 10 kpi surface finish while 16 rows of knives will produce about a 5 kpi surface finish. The goal of increasing the kpi number has generally been achieved by adding more cutterhead knives and/or increasing the speed of the cutterhead but limits to the number of knives and cutterhead RPM are being approached.
Three general types of cutterheads are in common use on high speed planers. The salient features and limitations of each design are as follows: